AND DISPERSION IN ICELAND SPAR. 
447 
The agreement between the two results is striking, and seems to show that we may 
assume the experimental results to be true with an error which is not greater than 
•00005. 
In my paper on a biaxal crystal I was able to show that the assumption of certain 
errors in the determination of the position of the plane of the prism with reference to 
the crystallographic axes led to results rather more in agreement with experiment 
than those obtained at first. 
In the present instance this is impossible, for any change in the position of the 
plane of the prism would produce effects of almost exactly the same amount in the 
values of /a for the lines C and g ; but the error we wish to correct in C is only half 
as great as that in g, and hence no change in the position of the plane can produce 
the required effect. 
But, again, the telescopes used were not perfectly achromatic for the red and violet 
rays. I found usually little or no difference in the position of the focus for the lines 
C and F, but there was an appreciable difference between C and g. 
If the collimator be focused so that the rays from the line C emerge parallel, those 
from g will be divergent. This may produce a variation in the angle of incidence 
between the waves C and g. 
For the prisms were so placed that by turning the table on which they rested 
without altering the position of the collimator, either face of the prism could be made 
a face of incidence. To secure this the edge of the prism passed nearly through the 
axis of the collimator, and in most positions of the prism the light from only about 
half the collimator lens reached it. 
A figure will make this clearer. S C (fig. 5) is the axis of the collimator, APB 
the prism, A P being the face of incidence. 
Fig. 5. 
Almost all the light incident on A P passes through the upper part, C E, of the lens 
of the collimator. If the prism be turned round K, a point in the axis S C produced, 
so that B Q becomes the face of incidence, then only the lower portion, C F, of the 
collimator lens will be used. 
Again, since the collimator is focused for red rays, they will be incident on the face 
A P in direction C P, and if P N be the normal to A P the angle of incidence will be 
C P N. 
3 m 2 
